Restriction fragments containing the coding sequence for the 5' terminus of the mouse ribosomal precursor will be selected and used for transcription studies. A close approximation of the initiation nucleotide will be made by S1 nuclease mapping (Berk and Sharp, Cell 12: 721-732 (1977)). This fragment as well as the similar sea urchin fragment will be transcribed in a mouse cell-free extract. Products will be characterized by sequencing the appropriate sized bands from amylamide gels. We will also compare sea urchin and mouse sequences around the cleavage processing sites to try to define signals recognized by processing enzymes.